Very low blood flow, or low "systemic perfusion," is typically due to low aortic pressure and can be caused by a number of factors, including hemorrhage, sepsis and cardiac arrest. The body responds to such stress by reducing blood flow to the gastrointestinal tract to spare blood for other, more critical organs. Thus, when there is a reduced flow of blood from the heart, the body directs a higher portion of blood to critical organs, such as the brain, which will not survive long without a continuous supply of blood, while restricting the flow to less critical organs, whose survival is not as threatened by a temporary large reduction in blood flow. For example, blood flow to the splanchnic vasculature which supplies the stomach and intestines, and also the esophagus and oral/nasal cavity, is drastically reduced when there is a reduced blood flow from the heart. For this reason, decreased blood flow to the splanchnic blood vessels is thus an indication of perfusion failure in a patient. Physicians commonly take advantage of this phenomenon by taking CO.sub.2 and pH measurements in the stomach and intestine to assess perfusion failure.
Assessment of CO.sub.2 concentration in the less critical organs, i.e., those organs to which blood flow is reduced during perfusion failure, has been useful in perfusion assessment. Carbon dioxide production, which is associated with metabolism, continues in tissues even during conditions of low blood flow. The concentration of CO.sub.2 builds-up in tissues experiencing low blood flow because CO.sub.2 is not rapidly carried away. This CO.sub.2 build-up (an increase in partial pressure of CO.sub.2 (PCO.sub.2)) in the less critical organs in turn results in a decrease in pH in nearby tissue. Therefore, perfusion failure is commonly assessed by measuring pH or PCO.sub.2 at these sites, especially in the stomach and intestines. For examples of catheters used to assess pH or PCO.sub.2 in the stomach or intestines, see, e.g., U.S. Pat. Nos. 3,905,889; 4,016,863; 4,632,119; 4,643,192; 4,981,470; 5,105,812; 5,117,827; 5,174,290; 5,341,803; 5,411,022; 5,423,320; 5,456,251; and 5,788,631.
The inventors have found that increases in PCO.sub.2 may be measured throughout the body, including in accessible organs and tissues fed by splanchnic vessels, and used to assess perfusion failure. For example, the inventors have found that a useful measurement of perfusion failure can be obtained by measuring CO.sub.2 in the upper respiratory/digestive tract. In U.S. Pat. No. 5,579,763, a method is described that can be used to accurately assess perfusion failure by measuring PCO.sub.2 in the patient's esophagus, rather than in the less accessible stomach and/or intestine as previously practiced in the art. Tests showed that measurements of PCO.sub.2 in the esophagus are closely correlated with aortic pressure, and, furthermore, that measurements made in the esophagus are even more closely correlated to aortic pressure than measurements of CO.sub.2 in the stomach. More recently, in co-pending, commonly assigned U.S. Ser. No. 09/160,224, the inventors further showed that PCO.sub.2 measurements in a patient's mucosal tissues (e.g., mouth, nasal mucosa, and throat) are also closely correlated to aortic pressure. As disclosed in U.S. Ser. No. 09/160,224, the CO.sub.2 sensor may be placed at a site within the oral-nasal cavity (e.g, under the tongue at a site in contact with the tongue or the floor of the mouth) where it effectively measures CO.sub.2 in the tissue. Since carbon dioxide can readily pass through mucosal surfaces, CO.sub.2 generated by metabolic activity occurring in tissue below the mucosal surface that is not carried away by blood flow readily migrates through the mucosal surface, where its build-up provides a good measure of perfusion failure. Placement of a CO.sub.2 sensor adjacent a mucosal surface of the upper respiratory/digestive tract thus provides a very good quantification of perfusion failure at all times, including the most critical minutes after the onset of perfusion failure when treatment is likely to be most effective. Thus, mucosal measurements of tissue perfusion can be used to assess perfusion failure in patients.
However, PCO.sub.2 and pH are indirect measures of blood flow in tissue, being based upon the build-up of metabolites that result from poor perfusion. In addition, measurements of pH may be complicated by the presence of saliva, food, or stomach acids. CO.sub.2 measurements may be affected by ambient CO.sub.2, and, since they depend on equilibration with tissue CO.sub.2 levels, are slow. Thus, there is a need for a more direct method for measuring blood flow in a tissue, to more accurately assess perfusion failure and to monitor the effectiveness of methods taken to increase perfusion, e.g., blood infusion or the like.